This invention relates to a printer and, more particularly, to a printer for printing an image on a recording medium by using a line-type printhead.
As a typical conventional ink-jet printer, a serial printer is available, which is designed to sequentially print images by moving the printhead laterally with respect to a conveying direction of a printing paper. As a printer using a line-type printhead having heating elements corresponding to one line, a thermosensitive-transfer printer using a thermal head or the like is well-known.
Such a printer is generally constituted by a formatter for bitmapping PDL data sent from a host computer into an image to be actually printed, a mechanical portion of the printer, and an engine for controlling the mechanical portion.
According to the above arrangement, a small-capacity memory can be used as an image memory (bitmap memory) for storing bitmapped data in a printer for printing an image on a label or card. However, a large-capacity memory is required in a printer for printing an image on a large label or performing a print operation by double buffer print control. In general, the basic arrangements of the above two types of printers are the same except that the image memories have different capacities. For this reason, if the capacity of an image memory is set to be small to limit the application of a printer, a print operation cannot be performed for a large label. In contrast to this, if a large-capacity image memory is integrated in a printer which is used to print an image only on a small label, some memory area becomes an unused memory area, wasting the expensive memory.
In general, in the above formatter, when image data is to be read out from the image memory storing a print image and sent to the engine to be printed, read control of image data is an important factor to increase the print efficiency.
In a color printer or the like having a plurality of line-type printheads, in particular, the image memory is divided into a plurality of areas in correspondence with the respective printheads, and the read address of print data to be transferred to each printhead is switched and output for each page. Such an address switching timing must be changed in accordance with the length of printing paper, the intervals of the printheads, and the like. It is very difficult to perform read control in consideration of all these conditions.
In the ink-jet printer using the above line-type printheads, when, for example, images of different colors are to be printed in units of lines by using the printheads for Y, M, C, and K (having about 1,400 printing elements; 1 element=1 dot), the total amount of print data to be transmitted from the formatter to the respective printheads to print one line corresponds to four lines (about 5,600 dots=700 bytes (in the case of binary data)). That is, this printer needs a higher data transfer rate than a conventional monochrome line printer. In addition, the time required to read out data from the bitmap memory storing such print data must be shortened. Furthermore, such a bitmap memory needs to have a large capacity to store print data for the four colors. In order to reduce the memory cost, a DRAM requiring a refresh operation is generally used.
In reading out print data from such a large-capacity bitmap memory, therefore, a DMA (Direct Memory Access) transfer method is usually employed in order to shorten the time required to read out data, and the refresh operation of the memory must be ensured. Since an increase in print speed is required, the time required to read out print data is limited. If a high-speed full color print operation is performed under this condition, the time required for DMA transfer prolongs with an increase in the amount of data. This might result in a failure of a refresh operation and a destruction of the contents stored in the memory. If priority is given to the refresh operation of the memory, print data may not be transferred in time.